Balloon mounted stent and method therefor

ABSTRACT

The present invention concerns a mechanism for securely mounting an expandable stent onto a balloon catheter for intraluminal delivery within a patient. The stent is secured to the balloon of the delivery catheter by forming the surface of the balloon to project into the interstices of the stent. To mount the stent it first is slid over an evacuated and wrapped balloon while in its compact delivery diameter. A rigid tube then is placed over the stent and balloon assembly and the balloon is pressurized while a solvent is applied to the balloon. The rigid tube prevents the stent from expanding but allows the balloon to deform so that its surface projects slightly through either or both of the interstices and ends of the stent. The solvent causes the balloon material to take a permanent set into the stent such that once pressure is removed, the stent is interlocked with the surface of the balloon. The engagement of the stent with the balloon prevents unintended axial sliding of the stent with respect to the balloon during delivery through the vessel of the patient.

FIELD OF THE INVENTION

This invention relates to stents for placement in the human body anddelivery systems therefor.

BACKGROUND OF THE INVENTION

The use of intraluminal stents has had increasing acceptance and use inthe treatment of various medical conditions. Intraluminal stents can beused to maintain the patency of blood vessels and other body lumens,such as in the treatment of urological disorders, among others. The useof stents to maintain open a coronary or other artery after anangioplasty procedure has been performed has become a common practice.Over the past decade, various stent designs have been described andused. Many have been of the type in which the stent is in a tubularconfiguration that can be expanded from a relatively small diameteradapted for delivery through the vasculature (low profile) to a largerdeployed diameter by mounting the stent on the balloon of an inflatableballoon catheter and then advancing the catheter to place the balloonmounted stent at the intended site of deployment. The balloon then isinflated to expand the stent into engagement with the body lumen. Thestent maintains its expanded shape to support the body lumen. Theballoon then is deflated and the delivery catheter is withdrawn, leavingthe stent in place to support the body lumen. The configuration of suchstents typically may be considered as generally tubular in which thewall of the tube is defined by variously configured wires or a tubularmember that has been shaped to provide a plurality of interconnectedwire-like struts or wires. Illustrative examples of such stents may befound in U.S. Pat. No. 4,733,665 U.S. Pat. No. 5,104,404 (Wolff), U.S.Pat. No. 5,421,955 (Lau) and in International Publication WO 96/41591(Borghi), among others.

The design and placement of a stent can present a number ofconsiderations, particularly when the stent is to be used in anenvironment where it must be advanced through tortuous anatomy, as isnot uncommon in coronary angioplasty. The stent must have the ability tomaintain its tubular configuration when expanded in the body lumen inorder to maintain the body lumen open. It also should have sufficientlongitudinal flexibility, when mounted in its low profile on theballoon, so that it can be advanced through curved, sometimes tortuous,blood vessels in order to reach the deployment site. Additionally, theposition of the stent on the balloon should be maintained withoutshifting longitudinally, as the balloon mounted stent is advanced to thedeployment site. That can present particular difficulty when the stentand balloon to which it is mounted must be passed through sharply curvedtortuous anatomy or anatomy where the lumen has become narrowed orotherwise partly obstructed regions. Should the stent engage a bloodvessel in a sharply curved or somewhat narrowed region and consequently,shift longitudinally on the balloon as the catheter is advanced ormanipulated, the balloon, when inflated, may not expand the stent fullyalong the length of the stent. In some cases, the stent could bedislodged from the balloon without the ability to retrieve it.

The importance of maintaining the stent securely on the balloon has beenrecognized in the prior art. A number of approaches have been proposed.Some simply slide the stent onto the balloon and crimp it tightly aboutthe balloon. That may risk damage to the balloon or the stent and alsomay result in a relatively longitudinally stiff stent. Other stentretention devices have been proposed including the use of a sleeve tooverlie the stent during advancement, with the sleeve being retractedwhen the stent is to be deployed. Still other approaches have includedthe use of end caps mounted on the catheter and adapted to temporarilyengage the ends of the stent while permitting the stent ends to releasewhen the stent is expanded as is shown in U.S. Pat No. 4,950,227(Savin). Such devices and techniques generally have required compromisesof one or more desirable features of the stent. The desirable lowprofile or longitudinal flexibility may be compromised as well as thesecurity of retention. It would be desirable to provide a balloonmounted stent and method for such mounting that would reduce the extentof compromise of desirable characteristics of the stent and its deliverysystem.

SUMMARY OF THE INVENTION

In accordance with the invention, a stent and balloon catheter assemblyis provided in which after the stent is mounted on the balloon that hasbeen wrapped about the catheter shaft in a relatively tight, lowprofile, the balloon is molded against the stent to secure the stentdirectly to the balloon. The outer diameter of the stent is maintainedduring the process, as by placing a rigid retaining tube about thestent. The interior of the balloon then is pressurized sufficiently sothat, although wrapped in a low profile configuration, it will tend toexpand radially outwardly against the stent. Concurrently with suchpressurization, polymeric material of the balloon is softened, as byheating, exposure to a solvent or other means for softening the balloonsufficiently to enable it to be molded. The balloon material may besoftened by any appropriate means including heat or solvents. The softmoldable condition of the balloon is maintained sufficiently to enableportions of the balloon to expand and mold against the ends of the stentor into interstices along the length of the stent, or both. Portions ofthe balloon thus are expanded to a radius that is greater than theradius of the inner lumen defined by the stent. The balloon ismaintained under conditions of pressure and softened condition for apredetermined time to assure the balloon will maintain its molded state.In the resulting assembly, the stent is interengaged with portions ofthe balloon to provide substantial resistance to shifting of thelongitudinal position of the stent along the balloon. The resistance tolongitudinal shifting created by the interengaged portions supplementsfrictional engagement between the stent and balloon serving to resisttheir relative axial movement. The balloon, being longer than the stentmay expand beyond the ends of the stent toward the inner diameter of theretaining tube to form a shoulder that engage the ends of the stent toprovide resistance to axial movement. The balloon expansion at the endsof the stent also reduces the abruptness of the transition between theballoon and the end of the stent to reduce the risk of snagging thestent on the guide catheter or vessel.

A general object of the invention is to provide an improved system formounting a stent securely onto the balloon of a stent delivery catheterwithout increasing the profile of the balloon and stent combination.Also among the objects of the invention are to provide an improvedballoon-stent combination in which the stent is secured on the balloonof a stent delivery catheter by interfering engagement of the stentdirectly with the balloon; to provide a balloon catheter and mountedstent adapted to pass through tortuous anatomy with reduced risk of thestent being displaced from its initial position on the balloon; toprovide an improved technique for mounting a stent on the balloon of astent delivery catheter; to provide a technique for effecting directengagement between a stent and the balloon of a delivery catheter inwhich the stent is securely mounted on the balloon without use ofsupplemental retaining or restraining devices; to provide an assembly ofa stent mounted securely on the balloon of a balloon catheter withoutsignificantly affecting the longitudinal flexibility of the assembly;and to provide a method for mounting a stent on the balloon of a stentdelivery catheter in which some portions of the balloon project radiallybeyond the radius defined by the inner surface of the stent.

DESCRIPTION OF THE DRAWINGS

The foregoing and other objects and advantages of the invention will beappreciated more fully from the following further description thereofwith reference to the accompanying drawings wherein:

FIG. 1 is a diagrammatic illustration of a stent delivery catheterhaving a stent mounted on the balloon of a delivery catheter;

FIG. 1A is cross-sectional view of the catheter shown in FIG. 1 as seenalong the line 1A--1A;

FIG. 1B is a digrammatic illustration of the distal end of the cathetershowing the configuration of the balloon, when inflated, and the shaftof the catheter;

FIG. 2A is a diagrammatic transverse cross-sectional illustration of theballoon region of a balloon catheter with the balloon having beenevacuated to form radially extended wings;

FIG. 2B is a diagrammatic illustration similar to FIG. 2A in which thewings of the evacuated balloon have been wrapped about the cathetershaft to a low profile configuration;

FIG. 2C is a diagrammatic illustration similar to that of FIGS. 2A and2B but in which the balloon has been inflated;

FIG. 3 is a diagrammatic illustration of a wrapped balloon and a stentmounted on a loading tube in readiness to be placed on the balloon;

FIG. 4 is an illustration of the balloon region of the stent deliverycatheter with the stent having been located on the balloon and with atubular restraining member being advanced over the assembly of the stentand balloon;

FIG. 5 is a diagrammatic, longitudinal sectional illustration of theinterfacing region between the balloon and the stent as seen along theline 5--5 of FIG. 1 in which the configuration of the interfacing regionbefore pressure molding is shown in solid lines, and the configurationof the interfacing region after pressure molding is shown in phantom;

FIG. 5A is a diagrammatic longitudinal sectional illustration of themanner in which the balloon can be molded against the ends of a stent toform shoulders engageable with the ends adapted to resist longitudinalmovement of the stent on the balloon;

FIG. 6 is a diagrammatic transverse section through the stentillustrating the relationship of the inner radius, outer radius andvirtual wall thickness defined by the stent; and

FIG. 7 is a transverse sectional illustration of the balloon mountedstent as seen along the line 7--7 of FIG. 5 showing the manner in whichportions of the balloon protrude radially beyond the inner radius of thestent and into engagement with radially extending surfaces of the stent.

DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENT

The invention, for convenience, is described in the context of a stentadapted for delivery in conjunction with an angioplasty procedure,although it should be understood that the invention may be practiced inconnection with stents and catheters adapted for other medicalprocedures. In the preferred embodiment, the stent delivery catheterincludes a catheter shaft 10 having at least one lumen 12 for inflationof a balloon 14 mounted at the distal region of the catheter shaft 10.As shown in FIG. 1B, the balloon may be configured to have a generallycylindrical, uniform diameter central section 11 and a pair of outwardlytapering end cones 13, each of which terminates in a neck 15 by whichthe balloon is attached to the catheter shaft 10. The portion of thecatheter shaft 10 that extends through the balloon may be provided witha pair of longitudinally spaced radiopaque markers 17. The markers maybe disposed on the shaft 10 at approximately the regions where the endcones 13 merge into the central balloon section 11.

The catheter shaft also may have a guidewire lumen 16 to enable thecatheter to be advanced and guided to the deployment site along aguidewire 18. The proximal end of the shaft 10 is provided with afitting 20 that includes a connector 22 by which the inflation lumen 12can be connected to a balloon inflation device (not shown). While adilatation catheter intended for angioplasty may be used in the practiceof the present invention, it is not necessary that the balloon 14 besuitable for an angioplasty procedure as it need only be capable ofretaining and expanding the stent in a delivery procedure that isseparate from the angioplasty procedure. However, the balloon of thestent delivery catheter of the present invention should havecharacteristics that enable it to be molded into engagement with theinterstices or other radially extending surfaces of the stent. Forexample, in the illustrative embodiment, the balloon may be thatdescribed in U.S. Pat. No. 5,500,180 (Anderson et al.).

In accordance with the invention a stent 24 having a generally tubularconfiguration is mounted on and about a balloon 14. The stent may besomewhat shorter than the cylindrical central section 11 of the balloonto assure that when the balloon is inflated within the patient to deploythe stent, the stent will be expanded along its length by the balloon.

The invention may be practiced with any balloon-expandable stent. Forpurposes of illustration, the invention is described as being used inconnection with a stent described in International Patent Application,Publication WO 96/41591 (Borghi), the disclosure of which is herebyincorporated by reference, in its entirety. Other examples of balloonexpandable stents include those disclosed in U.S. Pat. No. 4,733,665(Palmaz), U.S. Pat. No. 5,041,126 (Gianturco) and U.S. Pat. No.5,104,404 (Wolff), the disclosures of which also are hereby incorporatedby reference, in their entireties. In the illustrative embodiment, thestent is characterized by an arrangement of wire-like hoops 23 connectedto a spine 25 that, taken together, may be considered to define avirtual cylindrical tubular shape. The reticulated wires or struts 26that form the stent are arranged to define spaces 27 along the length ofthe stent, the configuration and orientation of which may vary as afunction of the geometry and design of the stent. The tubular stent thusmay be considered as defining an inner cylindrical radius R_(i) and anouter cylindrical radius R_(o), the difference between the two radiidefining what may be considered as the wall thickness T of the stent(FIG. 6). The stent may be formed by a number of techniques familiar tothose skilled in the art and may be formed from a variety of materials,including metals such as stainless steel or nickel titanium alloys(e.g., nitinol), polymeric or bioabsorbable materials. The structure andmaterial of the stent should be capable of enabling expansion from itslow profile configuration, in which it is adapted to be advanced throughthe vasculature to the deployment site, to an expanded diameter in whichit can engage and support the wall of a blood vessel or other bodylumen, as appropriate.

The stent is intended to be mounted on a balloon that itself has beenconfigured in a low profile. As shown in FIGS. 2A-2C, a common techniquefor configuring the balloon in a low profile is first to evacuate theballoon to cause the balloon to assume a cross-sectional shape as shownin FIG. 2A in which the balloon is collapsed and defines a plurality ofradially extending flat wings 28. While maintaining the evacuationpressure, the wings of the balloon are wrapped closely about thecatheter shaft in one of several compact configurations to maintain alow profile about the shaft, such as an S-shaped configuration shown inFIG. 2B.

With the balloon in its wrapped, low profile configuration, the stentthen is loaded onto the wrapped balloon as suggested in FIG. 3. Thestent may be positioned on the balloon so that the ends of the stent aredisposed adjacent and between the radiopaque marker bands on thatportion of the catheter shaft that passes through the balloon. Tofacilitate loading, the stent may be mounted about and crimped onto athin wall polymeric tube, shown in phantom at 29 that, in turn, can beslipped over the balloon. The tube 29 is advanced to position the stentin the desired location relative to the balloon with the end cones 13protruding beyond the ends of the stent. Then, while holding the stentin the desired position relative to the balloon, the tube 29 iswithdrawn, leaving the stent in the desired longitudinal position aboutthe balloon. The stent may be crimped slightly by hand or with acrimping tool to achieve a snug fit about the balloon.

A relatively rigid tube 30 then is placed over the stent. The tube 30may be formed from a suitably rigid tubing such as a polymercommercially available under the trade designation KYNAR and is intendedto restrain the stent from radial expansion during the remainder of thestent mounting process. The tube 30 should be of sufficient length tocompletely cover the stent and the balloon and may be of the order oftwice as long as the stent. The tube 30 should be dimensioned such thata relatively small clearance exists between the inside diameter of thetube and the outside surface of the stent as, for example, on the orderof 0.001"-0.003" to facilitate placement of the tube 30 over the stent.The balloon then is pressurized by connection of a suitable device tothe balloon inflation leg of the proximal fitting. In the illustrativeexample, the balloon may be pressurized with a gas, such as air ornitrogen, within the range of 6 to about 20 bar, preferably about 10bar. Although the balloon has been wrapped tightly in a low profileconfiguration, the wrapping cannot completely seal off all minute flowpassages through the balloon. Consequently, it is possible to applysufficient pressure within the wrapped balloon to cause portions of theballoon to press radially outwardly against the stent.

In the illustrative embodiment, the assembly of the pressurized ballooncatheter, stent and molding tube 30 then is heated to raise thetemperature of the balloon material sufficiently to allow it to yieldand be molded around the ends and against the interior of the stent. Atemperature in the range of 50°-70° C., preferably 65° C., under theabove pressures has been found to be sufficient to cause the balloon tobecome molded to the stent. Although the expansion of the balloon may beslight, it is sufficient to cause some portions of the balloon to expandto a radius that is greater than the inner radius R_(i) of the stent sothat those portions of the balloon may be considered as projectingradially outwardly into the region of the virtual wall thickness T. Theconditions of temperature and pressure are maintained sufficiently longto assure that the balloon will set and remain in that configurationwhen the balloon is cooled and pressure is removed. FIG. 7 is atransverse sectional illustration of the balloon mounted stent after theballoon has been molded and expanded radially outwardly so that portionsof the balloon, indicated at 14' protrude radially outwardly beyond theinner radius R_(i), of the stent as defined by the struts 26. FIG. 5 isa longitudinal illustration of the balloon and stent of FIG. 7. FIG. 5Aillustrates, diagrammatically and in enlarged scale, the region of themolded balloon with the end of a stent as confined within the retainingtube. The shoulder thus formed enables the balloon to engage directlythe ends of the stent to resist longitudinal movement of the stent onthe balloon. This embodiment may be used with any type of stent,including stents that have few or no interstices when in the low profileconfiguration.

Applicants have found that, in the illustrative embodiment, applyingheat and temperature for a time of approximately 30-90 seconds,preferably about 60 seconds, sufficiently molds the balloon to thestent. A brief cooling period of approximately one minute immediatelyfollowing the heating period, while maintaining pressure at 10 bar alsomay help set the balloon in its molded configuration. Although theballoon and stent may expand slightly to contact the inside of the tube30 while pressure is applied, slight elastic recoil of the stent afterthe inflation pressure is removed results in a slight clearance on theorder of 0.0005" between the tube and the stent. The restraining tube 30then is easily removed and the stent and balloon may be recompressedabout the catheter shaft by hand, or with a suitable tool to reduce theslight expansion that occurred during the balloon/stent engagementprocess. The delivery catheter with premounted stent then may bepackaged in readiness for use in a procedure.

It should be understood that although, in the illustrative embodiment,the balloon material is softened by the application of heat, otherprocedures may be employed to soften the balloon in the practice of theinvention. For example, the balloon material may be softened, whereappropriate, with a solvent sufficiently and under conditions to enablethe balloon to be molded under the influence of the pressuredifferential from the interior to the exterior of the balloon.

From the foregoing, it will be appreciated that the invention provides aballoon mounted stent and a method for mounting the stent on the balloonin which there are no stent retaining components to increase the profileof the assembly while maintaining the stent in a secure position on theballoon by direct, interfering engagement of the stent with the balloon.The invention provides a balloon catheter and mounted stent in which thelongitudinal flexibility is not adversely affected and that can passthrough tortuous or restricted anatomy with reduced risk of the stentbecoming dislodged or displaced from its position on the balloon. Anumber of advantages thus are achieved with minimal compromise. Itshould be understood, however, that the foregoing description of theinvention is intended merely to be illustrative thereof and that otherembodiments, modifications and equivalents within the scope of theinvention may be made by those skilled in the art.

Having thus described the invention, what we desire to claim and secureby Letters Patent is:
 1. A method for mounting a stent on a balloon of acatheter comprising:configuring the balloon in a low profile; placing aballoon expandable stent on the balloon; expanding portions of theballoon radially outward to a radius greater than the inner radius ofthe stent to resist longitudinal movement of the stent relative to theballoon while in said low profile; wherein the step of expandingportions of the balloon radially outwardly comprises molding at leastpart of the balloon radially outwardly into engagement with and againstthe inner contour of the stent by concurrently applying pressure to theinterior of the balloon and softening the balloon material sufficientlyto enable it to be molded against the inner contour of the stent underthe influence of said pressure, wherein the softening of the balloonmaterial is accomplished by applying a solvent to the balloon material;and maintaining the softened balloon in the pressurized condition,sufficiently to cause portions of the balloon to be molded radiallyoutwardly against and in conformity to at least a portion of the innercontour of the stent.
 2. A method as defined in claim 1 furthercomprising:while maintaining the pressure in the balloon, allowing theballoon to set to a molded configuration that includes a shoulderadjacent and engageable with at least one end of the balloon.
 3. Amethod as defined in claim 1 further comprising maintaining the diameterof the stent while the balloon is pressurized.
 4. A method as defined inclaim 3 wherein the step of maintaining the stent diameter comprisesplacing a restraining member about the exterior of the stent before theinterior of the balloon is pressurized.
 5. A method as defined in claim4 wherein the step of maintaining the diameter of the stent comprisesplacing a rigid tubular member over the stent.